Phytoplankton communities in the crater lakes of western Uganda, and their indicator species in relation to lake trophic status

The freshwater crater lakes of western Uganda represent an important natural resource for the region's rural communities, but their capacity to provide drinking-quality water and other ecosystem services is being threatened by rapidly intensifying human activity within their catchments. This study investigated the relationship between the composition of pelagic phytoplankton communities in these lakes and their trophic status, to assess how phytoplankton surveys with recognition of indicator species can assist in the assessment of water-resource vulnerability to human impact. For this purpose we analyzed 70 phytoplankton samples with linked environmental data from 26 crater lakes spanning the entire regional gradients of primary productivity and land-use intensity, with each lake sampled two to four times and during both dry and rainy seasons. At this equatorial location, nutrient concentrations and phytoplankton biomass (measured as the concentration of chlorophyll-a) showed no consistent seasonal pattern. In total we recorded 118 phytoplankton taxa from 72 genera, and from 20 to 54 taxa per lake. The phytoplankton community of most lakes and in most seasons was dominated by cyanobacteria (on average 78% of total cell counts) followed by green algae (17%) and diatoms (4%); euglenophytes, dinoflagellates, chrysophytes and cryptophytes together contributed less than 1% on average. Many species were found to have a wide distribution along the productivity gradient, which complicated characterization of phytoplankton communities typical of different trophic states. Cluster analysis revealed four groups of samples, of which two broadly represented the least and most highly productive lakes. The two other groups comprised a mixture of lakes with intermediate productivity, suggesting that phytoplankton composition in these lakes is structured by other factors in addition to nutrient availability. In total 25 taxa displayed a significant association with one of these groups, but only 12 of them can be considered robust indicators of a particular trophic level; among these, the cyanobacteria Planktolyngbya limnetica and Microcystis aeruginosa qualify as strong indicator species for lakes with low and high productivity, respectively. Based on the distributional turnover between indicator species, we propose that the fresh Ugandan crater lakes comprise four trophic levels, of which the boundaries are situated higher up the chlorophyll-a gradient than those between the four classic trophic levels defined for temperate-region lakes. For practical use, Ugandan crater lakes with a chlorophyll-a concentration up to 6 mu g/L can be defined as oligotrophic; similarly, we situate the mesotrophic/eutrophic boundary at ca 20 mu g/L, and the eutrophic/hypertrophic boundary at ca 60 mu g/L.